What is B-shirt?

A t-shirt you get free when every you buy a case of beer. Usually given away as a promotion.

Frat Guy One: Hey, did you get the beer?
Frat Guy Two: Yah, and a free b-shirt to go with it.

B-shirt - meme gif

a t shirt that was worn by a fat person for so long the belly part has inflated and given the t shirt the shape of a b.

person 1: let's get six third pounder and four large fries for dinner.
person 2: dude now i get why you always wear b shirts